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This Week in Science

Battle of the Sexes

Much research on sexual selection has focused either on intrasexual competition between males or on female mating preferences. Clutton-Brock (p. 1882) reviews recent studies which show that intrasexual competition between females and male preferences for particular categories of partners are also common and can generate secondary sexual characters that are more highly developed in females. Sexual selection may now need a new conceptual framework that incorporates the effects of intrasexual competition and mating preferences in both sexes.

Aging in Glasses

Glasses age over time, but an understanding of the structural rearrangements that underlie these processes in small-molecule systems is difficult because it is not possible to track the motion of individual molecules, and the overall changes in ordering may be small. Schall et al. (p. 1895; see the Perspective by Falk) tracked the motions of colloidal glasses under small strain motions using confocal microscopy. Localized zones form where the colloidal particles undergo irreversible shear transformations. Further analysis revealed how the sheared colloidal glasses are activated and how they interconnect into networks.

Mechanics of Amyloid Fibrils

Amyloid structures associated with a number of diseases form from a wide range of unrelated polypeptides and show intriguing but poorly understood physical properties. Using atomic force microscopy to image a set of protein fibrils, Knowles et al. (p. 1900) measured the local mechanical properties and correlated these results with coarse-grained atomistic molecular simulations. By controlling the hydrogen bonding, fibril stability can be altered or reinforced and used to offset specific side-chain interactions.

Heating Mars with SO2

Evidence that Mars had liquid water on its surface when the planet was young implies that air temperatures were above the freezing point of water, unlike today. If these conditions were mainly the result of greenhouse gas heating by CO2, then the partial pressure of CO2 should have been high enough that carbonate minerals would have formed—yet these minerals have not been observed on Mars' surface. Halevy et al. (p. 1903) propose that volcanically degassed SO2, emitted under more reducing conditions along with H2S, would in combination with CO2 have pushed temperatures over the threshold required by liquid water. Dissolved SO2 also would have acidified the oceans enough to prevent carbonate minerals from forming. A similar mechanism operating on Earth may explain the absence of carbonate rocks from the Archean.

An Exercise in Quantum Geometry

Quantum computation relies on the ability to coherently manipulate the quantum state of qubits. However, unavoidable coupling to the environment gives the qubit a finite lifetime. It has been proposed that the use of a geometric phase (or Berry's phase, a topological phase that accumulates as an object traverses a path) should be more robust to the effects of decoherence. Leek et al. (p. 1889, published online 22 November) describe the observation of this geometric phase in a superconducting qubit, which they claim might bring fault-tolerant quantum computation a step closer.

Carbon Nanotube Fiber Fabrication

By twisting together even short segments of string or straw, a strong fiber or rope can be formed as long as the starting material is long enough to properly twist together and is compressed sufficiently to ensure stress transfer between the segments. In theory, carbon nanotubes (CNTs) should be able to form very strong fibers because of their impressive intrinsic properties. Koziol et al. (p. 1892, published online 15 November) show that they can generate CNT aerogels and directly spin them into strong and stiff fibers. Further densification by treatment with acetone ensured maximal stress transfer between adjoining fibers. The authors compared the strength and stiffness of these fibers to other CNTs and commercial materials such as Kevlar.

Slippery Serpentine Sheets

Serpentinite layers that coat the top of sinking lithospheric slabs have been thought to play a role in subduction zone earthquakes because these layers become heavily deformed. Hilairet et al. (p. 1910) deformed the serpentine antigorite at high pressures and temperatures and found that it has unusually low viscosity that could account for postseismic deformations after large earthquakes within subduction zones. This property may also enable subduction initiation and may govern convection within subduction zones.

Planet of the Beetles

Beetles represent more than 20% of all described species, although relationships within the order are still speculative. Hunt et al. (p. 1913) reconstructed the phylogeny of >80% of recognized beetle families and identified previously unknown relationships for many groups. By performing analyses of diversity patterns across the entire order, the authors estimated that diversification of major beetle groups may have occurred in the Jurassic, earlier than previously thought.

Induced Human Pluripotent Stem Cell Lines

Embryonic stem cells can grow for an unlimited time and can turn into essentially every type of cell, which makes them an ideal candidate for regenerative medicine (see the Perspective by Cibelli). However, their applications are hindered by potential problems such as immune rejection and ethical concerns about their origin. Yu et al. (p. 1917, published online 20 November; see the 23 November news story by Vogel and Holden) report a method to derive pluripotent stem cells from human fibroblasts. By introducing four genes (OCT4, NANOG, SOX2, and LIN28) into human fibroblasts, stem cells sharing essentially all of the features of human embryonic stem cells were obtained. Hanna et al. (p. 1920) used a method to reprogram mouse cells to a pluripotent state that is similar to that of embryonic stem cells to generate so-called mouse-induced pluripotent stem cells, or iPS cells, from mice with humanized sickle cell anemia. The iPS cells were derived from a skin biopsy of this mouse model, and the genetic defect was eliminated by gene correction. These cells were directed to differentiate into hematopoietic progenitors and then transplanted into donor sickle cell mice, which rescued the disease phenotype.

Orphanages and Fostering in Romania

Studying a time when Romania did not have a foster care system for abandoned children, Nelson et al. (p. 1937) constructed a foster care system and followed the outcomes of children randomly selected to receive orphanage care or foster care. The results show that children moved to foster care homes had better cognitive outcomes, and that the earlier the child was moved out of the orphanage, the better the outcome. In a Policy Forum, Millum and Emanuel discuss the ethical issues posed in such studies and the safeguards needed when parents and guardians are not available to give consent.

Histones Coming and Going

Eukaryotic nuclear DNA is packaged into nucleosomes, which must be removed to allow replication of the genome and then reassembled onto the two newly synthesized daughter strands. Coordination of this removal and deposition process must occur at each replication fork. Groth et al. (p. 1928) show that this equilibrium is regulated by the histone chaperone antisilencing function 1 (Asf1). Asf1 exists in a nuclear pool associated with the MCM2-7 complex—the putative replication helicase—and histones H3 and H4. Thus, Asf1, through its interaction with the helicase and parental histones, coordinates template unwinding and removal of nucleosomes ahead of the replication fork as well as their deposition behind the fork.

Rapid Reorganization of Neuronal Connectivity

Reorganization of the brain motor cortex output is thought to involve excitability changes within the cortex per se, while the effect of individual output neurons on muscle activity remains constant. However, Davidson et al. (p. 1934) found that throughput from single motor cortex neurons to muscles can vary so much as to be absent during some behaviors and present during others. In particular, effects not present during a simple movement often appeared when a monkey was rewarded specifically for discharging a neuron and activating a muscle simultaneously. Rapid changes thus occur at subcortical levels, including the monosynaptic connections from motor cortex neurons to spinal motoneurons.

Early Earth Differentiation

Neodymium (Nd) isotope ratios in ancient rocks probe the period of differentiation of the early Earth. By measuring high-precision Nd isotopic data for rocks that are 3.64 to 3.85 billion years old from southwest Greenland and Western Australia, Bennett et al. (p. 1907) were able to date directly the formation of chemically distinct silicate reservoirs during the first 40 to 60 million years of Earth history, near the time of core segregation. They find excesses of 142Nd compared with modern terrestrial compositions, primitive meteorites, and the Moon. The differences in 142Nd signatures of coeval rocks from the two most extensive Eoarchean (more than 3.6 billion years ago) crustal relicts reveal large-scale chemical dichotomies in the Earth's mantle that persisted for at least the first billion years of Earth history. Temporal variations in 142Nd signatures track the subsequent incomplete remixing of these very early formed mantle chemical domains.

G Protein Interactions Revealed

Receptor-linked heterotrimeric guanine nucleotide-binding proteins (G proteins) often produce signals in cells by activating small guanosine triphosphatases like RhoA. Lutz et al. (p. 1923) analyzed the crystal structure of the activated G protein α subunit Gαq with an interacting portion of the guanine nucleotide exchange factor p63RhoGEF (an activator of Rho) and Rho, which suggests a model for how the GEF is activated. Mutagenesis studies supported the model in which interactions of domains within p63RhoGEF appear to cause auto-inhibition of the enzyme. Interaction of this same inhibitory region of p63RhoGEF with the activated Gαq subunit instead appears to relieve the inhibition of the GEF, which then leads to activation of RhoA.

On Again, Off Again

The 3′-untranslated regions of eukaryotic messenger RNAs (mRNAs) frequently contain regulatory sites, such as AU-rich elements (AREs) and micro (mi) RNA binding sites, the latter of which generally down-regulate gene expression by repressing translation, by promoting mRNA degradation, or through both processes. The ARE in the gene for tumor necrosis factor-α (TNFα) activates translation in nonproliferating cells, under which conditions it associates with the AGO2 protein and fragile X mental retardation-related protein 1 (FXR1), which also associate with miRNAs. Vasudevan et al. (p. 1931, published online 29 November; see the Perspective by Buchan and Parker) now show that human miR369-3 binds the TNFα ARE, recruits AGO1 and FXR1, and is required for the up-regulation of translation seen in nonproliferating cells. In contrast, miR369-3 represses translation of the same mRNA in proliferating cells. Similar results pertain for three other miRNAs in their regulation of reporter constructs, suggesting that miRNA function—rather than being constitutively repressive in nature—is dictated by the cell cycle.